How do you overcome the hose transfer when you want a quick section change do you use a John Guest inline connector to save trying to thread the hose down the pole or stick a double barb in the pipe?
As you've answered for yourself, if I do this I unplug the hose from the push fit valve, slide sections on and off and re-connect. If you don't have a push fit valve, we do sell straight 8mm to 8mm JG push fit connectors which can be added to your hose.
To be honest, I used to do this when I only had the 30ft SL-X but now I have the smaller selection in the van, I don't bother doing it I just grab a smaller pole.
I've got another one 
Can a thirty foot SLX be upgraded to a 35' SLX by buying an extra section? Would it be a top or bottom section and what would it cost?
Yes, it can be by buying a spare base section for a 35ft SL-X. In fact this section would extend either of the previous SL-X models. Give us a few weeks and we'll have the full range of spares available on the website.